1. Field of the Invention
The present invention relates to a zoom lens system which has a super-wide field angle and is to be used in cameras, etc.
2. Description of the Prior Art
Most of the conventional wide angle zoom lens systems which are used in silver salt cameras, etc. have field angles on the order of 63.degree. to 84.degree. at wide positions of these zoom lens systems. Zoom lens systems having field angles larger than the order mentioned above exhibit remarkable effects for photographing even when the lens systems have low vari-focal ratios since these zoom lens systems allow variations of field angles which are large for variations of focal lengths thereof.
As a zoom lens system having a field angle which is large as described above, there is known the zoom lens system disclosed by Japanese Patent Kokai Publication No. Hei 2-296,208. This zoom lens system comprises, in order from the object side, a first lens unit having a negative refractive power, a second lens unit having a positive refractive power, a third lens unit having a negative refractive power and a fourth lens unit having a positive refractive power, and has a high aperture ratio of an F number of F/2.8. However, each of the lens units is composed of a large number of lens elements, and a lens element disposed at an object side location is an aspherical lens element, for correcting mainly distortion at the wide position, and can hardly be manufactured in practice.
Further, as a zoom lens system which has a simple composition, there is known the zoom lens system disclosed by Japanese Patent Kokai Publication No. Sho 60-130712. This zoom lens system has a simple composition consisting only of two lens units, i.e., a first lens unit having a negative refractive power and a second lens unit having a positive refractive power which are disposed in order from the object side. However, this zoom lens system allows an F number thereof to be varied by zooming and has an F number of F/3.5, or a low aperture ratio at a wide position thereof.